Microbial communities of the human gut are an essential component of normal health and development. In conjunction with host-specific factors, a complex network of cooperative and competitive microbial interactions shape the function, diversity, and structure of these communities. The nature, mechanism, and strength of these interactions, however, as well as the selective pressures they exert, remain difficult to quantify. A greater understanding of how gut-associated Bacteria interact, and the extent to which these interactions are distributed amongst different phylogenetic and functional groups of organisms, would provide valuable insight into the ecological and evolutionary forces shaping microbial communities of the human gut. Here we propose to thoroughly map competitive and cooperative interactions amongst two dominant gut-associated bacterial phyla-the Bacteroidetes and the Firmicutes. We will apply a novel sequencing-based approach to generate time-resolved quantification of co-cultured species and identify the genetic determinants shaping the outcome of interactions through the use of genome-wide transposon mutagenesis libraries. In this way, we seek to create a robust mechanistic framework for interpreting the ecological processes and selective pressures shaping microbial community assembly in the human gut. These are critical steps towards the ultimate goal of understanding, diagnosing, and treating dysbioses in human gut microbial communities.